In systems which are being increasingly automated, a tool change for a machine tool may involve the selection by a gripper of a tool from a tool magazine, the transfer of the selected tool to a tool holder, the engagement of the selected tool by the tool holder so that the latter continues to be gripped after the tool holder is moved from its position in which the tool is mounted thereon, the engagement of a tool no longer required by the gripper, the release of the engaged tool from the tool holder and the return of the removed tool to the magazine.
The tool holder can be one of a number of such tool holders mounted upon a turret.
The use of automated mechanisms of this type in machine tools nevertheless requires that precision and resistance to forces and stresses be insured in the connection between the tool and the tool holder, particularly when the tool is only a portion of the entire structure, i.e. a cutter and a tool head supporting same. In that case, the tool head may be connectable to a tool shaft which forms the holder and is referred to herein as a tool base holder. The latter can have a shaft portion extending away from the tool head.
In this case, the above described tool replacement need not involve the entire tool, but only the active part thereof, namely, the tool head which is formed with a cutting plate or the like.
This has the advantage that the manipulators and other devices for handling the tool head and effecting the exchange can be dimensioned somewhat smaller than would be the case if the entire tool had to be changed because of the small dimensions and relatively low weight of the tool head. The magazine itself may equally be of reduced size.
An arrangement in which a tool head can be mounted upon a tool base holder is described, for example, in German Patent Document 30 07 440. In this case, the tool base holder receives an axially movable tension rod which has a head actuating a locking mechanism within the tool base holder. This head end of the rod is turned toward the tool head and the mechanism can comprise radially outwardly displaceable locking pins which can be driven outwardly when the tension rod is driven in the opposite direction so that these pins engage in recesses in the tool head or carrier.
The tension is generated by a prestressed disk spring stack, i.e. a stack of Belleville springs, which can be braced against a shoulder or flange at the opposite end of the tension rod.
In this system, to release the tool carrier from the tool holder, an external force must be applied to the tension rod, sufficient to overcome the spring bias applied thereto.
This releasing force or relief of the tension is generally applied from the exterior by mechanical, hydraulic or pneumatic means.
Advantageously, this clamping system allows very short tool replacement times to be realized, although during the tool replacement operation extremely high forces may have to be applied to the rod to overcome the force of the disk spring stack. The result is extreme force application to the tool holder and to the turret itself since the latter must resist the forces applied to release the tool carrier. As a consequence, the journals of the turret may be inordinately stressed and, if they are not dimensioned adequately to resist such stress, there is a rapid breakdown of the entire system. Even if the turret system is dimensioned to take up such stresses, however, the repeated application of high unbalnced forces can result in early failure.
To overcome this drawback at least in part, it has already been provided in the United Kingdom Patent Documents 21 63 379 and 21 54 479, to utilize a stress generating unit which applies a force via a hydraulic piston and a pressure transformer (pressure amplifier). Since each tool holder of a tool turret must include such a stressing unit, a system with a hydraulic piston and pressure transformer arrangement entails very high capital cost.
The hydraulic pressure transformer system for each tool must be separated from other systems or provided with separate circulation paths or cutoffs so that, upon repair or maintenance, there will be no total draining of the hydraulic medium of such systems. This again increases the capital and maintenance costs.
It is also known to provide mechanically actuated tool holders and to axially shift the tension rod by a nut utilizing a screw system. The rod is thus placed under compression or tension by the screw system. While such an arrangement is relatively insensitive to vibration and insures an effective locking, the serviceability is poor and it requires considerable manual work. It is not as amenable to automation as the hydraulic or clamping systems which have been previously described.